A fuel cell is an electrical cell that converts the intrinsic chemical energy of the chemical reaction between a hydrogen-containing fuel and oxygen directly into direct-current electrical energy in a continuous catalytic process. As in the classical definition of catalysis, the fuel cell should not itself undergo change; that is, unlike the electrodes of a battery, its electrodes ideally remain invariant. As compared to other energy sources, fuel cells provide advantages that include low pollution, high efficiency, high energy density and simple fuel recharge.
Proton Exchange Membrane Fuel Cells (PEMFC; also referred to as Polymer Electrolyte Membrane Fuel Cell) are a type of fuel cell, which are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines. First used in the 1960s for the NASA Gemini program, PEMFC's are currently being developed and demonstrated for systems ranging from 1 W to 2 kW.
PEM fuel cells use a solid polymer membrane as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons or gases, such as oxygen and hydrogen. The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:
Anode Reactions: 2H2=>4H++4e−
Cathode Reactions: O2+4H++4e−=>2H2O
Overall Cell Reactions: 2H2+O2=>2H2O
Compared to other types of fuel cells, PEMFC's generate more power for a given volume or weight of fuel cell. This high-power density characteristic makes them compact and lightweight. In addition, the operating temperature is less than 100° C., which allows rapid start-up. These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power and other applications. Other advantages result from the electrolyte being a solid material, compared to a liquid. The sealing of the anode and cathode gases is simpler with a solid electrolyte, and therefore, less expensive to manufacture. The solid electrolyte is also more immune to difficulties with orientation and has fewer problems with corrosion, compared to many of the other electrolytes, thus leading to a longer cell and stack life.
The performance of a proton exchange membrane fuel cell (PEMFC) is highly dictated by the proton conductivity of the polymer electrolyte membrane. A polymer electrolyte membrane, such as the widely used Nafion® (DuPont) used in fuel cells, requires sufficient amount of water to maintain their proton conductivity. The protonic conductivity increases with the increase of water content. The requirement of sufficient water supply also implies that it is not feasible to use temperatures above 80° C. to 90° C.
Basically, the process of water transport in the polymer membrane involves two pathways. One is that the water is dragged along with protons through the polymer membrane from the anode to the cathode by the electro-osmotic drag which increases with increasing current density and humidity (indicated in FIG. 1 with H++nH2O). The number of water molecules (n in FIG. 1) dragged with each proton is between 1 and 2.5. The poorer the fuel cell performance the more severe the electro-osmotic drag will be. The water drag from the anode to the cathode of the fuel cell is proportional to proton flow and thus this phenomenon increases at higher current density. The other process is that of the back-diffusion of water molecules (indicated in FIG. 1 with the phrase “back diffusion H2O”) from the cathode to the anode due to the concentration gradient which is built up by water produced at the cathode and the drive of the electro-osmotic drag. The phenomenon of water back diffusion across the polymer membrane from the cathode to the anode is usually dominant due to the water produced at the cathode. As the sole by-product of the hydrogen-oxygen reaction is water, which occurs at the cathode side (indicated in FIG. 1 with the phrase “H2O generated”), it is likely that flooding occurs at the cathode side and dehydration occurs at the anode side of the membrane.
Thus, the water balance in the membrane is a complicated issue. It is a major challenge for PEMFC technology. To avoid desiccation of the fuel cell, the traditional method of external humidification of the gases has been applied to practical fuel cell systems. To achieve enough hydration, water is normally introduced into the cell externally by a variety of methods such as liquid injection, steam introduction and humidification of reactants by passing the hydrogen and air through humidifiers prior to entering the cell. Humidification by the last method is relatively easy to handle and therefore, it is the most commonly used technique. In addition to externally introduced water, the water content of a polymer membrane in an operating fuel cell is dependent on several other factors, such as fuel cell operating conditions (temperature, pressure, flow rate and electrical load, etc.), properties of membrane (thickness) and electrode (composition).
However, the external humidification brings a burden to fuel cell systems, especially for those systems having constraints in size and portability. Two feasible methods of alternate humidification of membrane without external humidification were proposed: (1) self-humidifying polymer electrolyte membrane, (2) internal humidification.
Watanabe et al. (Watanabe, M., Uchida, H., Seki, Y. and Emori, M.; J. Electrochem. Soc.; 1996; vol. 143; no. 12; p. 3847-3852) first developed a self-humidifying membrane by recasting the solubilized Nafion ionomer and incorporating in it nano-sized platinum (Pt) and metal oxide particles. The nano-sized particles of Pt and oxides such as TiO2 or SiO2 are both highly dispersed in the thin electrolyte membranes. The platinum and added oxides provide a means for combining H2 and Oz into water, and then retaining the water in the hygroscopic oxides to maintain the water content in the membrane. The cell using the self-humidifying membrane reported by the researchers showed stable and high performance even under ambient pressure conditions when fed with hydrogen saturated with water at 20° C. and dry oxygen. The output of the cell reached 0.63 W/cm at 0.9 A/cm2. However, this method needs additional Pt in membrane and tedious membrane preparation steps for including those molecules in the membrane.
Several modeling and experimental studies revealed that PEM fuel cell operation with internal humidification using the water generated from electrochemical reaction and the self-water balance in membrane is feasible under restricted operating conditions with regard to gas flow rates and cell temperature or specific electrode/flow field design. For example, Büchi and Srinivasan (Büchi, F. N., Srinivasan, S.; J. Electrochem. Soc.; 1997; vol. 144; no. 8; p. 2767-2772) developed a model for predicting the possibility of operating a PEM fuel cell without external humidification of the gases and experimentally verified the model. In their experiment, a PEM fuel cell with a conventional MEA using Nafion® 115 had demonstrated stable long-term operation over a period of 1800 h at the cell temperature of up to 60° C. with dry reactant gases. However, it was found that the non-humidified cell still performed lower current density (at 0.6 V) than an identical cell with humidified reactants even if the air flow rate and temperature were correctly set. Recently, Chan et al. (Chan, S. H., Xia, Z. T., Wei, Z. D.; J. Power. Sour.; 2006; vol. 158; p. 385-391) have developed a model for a small non-pressurized non-humidified PEM fuel cell stack. In their model, the relationships between conductivity and water loading, water loading and relative humidity, and relative humidity and air stoichiometric number under different constant temperatures (25° C. to 40° C.) were established. The results showed that the air stoichiometric number strongly affects the conductivity of membrane which is performance related.
Due to the aforementioned problems which still exist in the prior art, a need exists for the development of fuel cells in which the water management is further improved for stable operation of fuel cells.